This invention relates to methods and apparatus for reducing NOx in engine exhaust and more particularly to method and apparatus for reducing NOx in engine exhaust using hydrocarbon to react with such engine exhaust.
As is known in the art, lean burn engines (e.g., diesel and DISI engines) provide great fuel efficiency compared to stoichiometric spark ignited engines at the expense of more complicated exhaust after-treatment. More particularly, one such after-treatment is the reduction of engine exhaust NOx. Lean NOx catalysts (ALNCs) are typically utilized to reduce tail pipe NOx emissions.
In a typical ALNC configuration, a reductant or reactant, e.g., urea or hydrocarbon, is introduced into the engine exhaust stream. In the case of a hydrocarbon (HC), the hydrocarbon is to react with the NOx in the engine exhaust stream and the reaction is facilitated in the catalyst. This NOx reduction arrangement is essentially an open-loop arrangement because a measurement of the effectiveness of the NOx reduction is not used to adjust the amount of reactant being introduced, or injected into the engine exhaust. This open-loop arrangement includes a look-up table which stores the relationship between the desired amount of hydrocarbon injection in accordance with engine speed, engine load, EGR level, catalyst temperature and space volume, inter alia. Typical injection strategies compute the HC quantity q1 to be injected as the product of a first function f1 (where f1 is a function itself of space velocity (SV), engine speed (RPM) and fuel quantity (fuel)) and a second function, f2, which is a function of catalyst temperature, Tcat. More particularly, q1=f1(SV, RPM, fuel)*f2(Tcat). Thus, f1 and f2 are determined a priori to thereby compute q1. The signal representative of q1 is used as the control signal for an HC injector.
It should be noted that f2 is a function of the optimum catalyst conversion (i.e., NOx reduction) temperature. Such function f2 is shown in FIG. 1. It is noted that f2 has a value of 0 for catalyst temperatures less than T_LOW and catalyst temperatures greater than T_HIGH. The function f2 is 1.0 between catalyst temperature T1 and T2, where the optimum conversion temperature T_CAT_OPTIMUM for the particular catalyst shown in FIG. 1 is between T1 and T2. Finally it is noted that the function f2 monotonically increases from 0 to 1 between T_LOW and T1 and monotonically decreases from 1 to 0 between T2 and T_HIGH. Finally, it should be noted that the function f2 shown in FIG. 1 is for the particular catalyst when such catalyst is green, or un-aged. The inventor has recognized that this function, and more particularly T_CAT_OPTIMUM, changes as the catalyst ages. Thus, while the function f2 may be accurate for a green, or un-aged, catalyst, this a priori determined function f2 is not accurate as the catalyst ages. Thus, the amount of HC added to the reaction may not be optimum as the catalyst ages. Further, because there is no NOx sensor downstream of the catalyst the NOx reduction effectiveness is not measured directly. That is, there being no measure of the NOx reduction effectiveness there is no feedback signal which may be provided to modulate or adjust the hydrocarbon injection process.
The inventor has discovered a method and apparatus which enables the development of a feedback signal indicative of the effectiveness of a reactant in reducing a substance reacted with such reactant without use of a sensor to detect the amount of un-reacted substance.